Webmail error: alway show THIS FOLDER IS EMPTY

Inbox folder's always show: THIS FOLDER IS EMPTY (horde and squirrelmail), But i can receive all email by outlook (in outlook config i choise "live copy messenger on servers"
I try close outlook and send a new mail but the inbox folder show THIS FOLDER IS EMPTY.
Please help me how to fix this?

PartnerNOC

Always leave a copy on the server is a hack that is not really part of what POP3 is all about. if you require a working model try configuring your mail client for IMAP instead, which does this as part of the specification.